rbd task: support xfstests repeat count

rbd task: support xfstests repeat count

This adds the ability to use the new repeat count argument to the
run_xfstests.sh script.  By default, the test suite will be run
once, but if a count is specified the script will execute the suite
that many times, but will only perform the setup (building the
tests, etc.) once.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>